CAUSAL is a 6 letter word that starts with C.

1. \Caus"al\, a. [L. causalis. See {Cause}.]
   Relating to a cause or causes; inplying or containing a cause
   or causes; expressing a cause; causative.
   
         Causal propositions are where two propositions are
         joined by causal words.                  --Watts.
   
   

2. \Caus"al\, n.
   A causal word or form of speech.
   
         Anglo-Saxon drencan to drench, causal of Anglo-Saxon
         drincan to drink.                        --Skeat.